Theoretical studies on the electronic and spectroscopic properties of Keggin-structure polyoxometalates α/β-[XM12O40]n- (X = Si, P; M = Mo, W)

The Keggin-structure compounds, alpha/beta-[XM12O40](-) (X = Si, P; M = Mo, W), are optimized by the HF and B3LYP methods, respectively. Though the geometry parameters optimized by the HF agree better with experimental values, the B3LYP calculations can present precise energies, vibrational frequencies and suitable geometries. The IPCM model is used to account for the solvent effect of water. Based on the HF optimized geometries, the absorption spectra of title compounds in the aqueous solution are obtained by the combination of CIS and IPCM methods, in which alpha-[PW12O40](3-) as an example is discussed in detail. The calculated 252 nm X(1)A(l) --> C(1)A(l) transition of such an anion is attributed to an O-b/c/d --> W (intra-framework W12O36) charge transfer, corresponding to 265 nm absorption in water observed in experiment. The transition property of the absorption clearly substantiates the clathrate model hypothesis that a Keggin anion may also be viewed as a neutral M12O36 cage that encapsulates the internal XO4n- subunit. (C) 2004 Elsevier B.V. All rights reserved.